This invention relates to a carbonaceous material capable of doping and de-doping lithium and a method for producing the same and also to a non-aqueous electrolytic cell having the carbonaceous material as a negative electrode.
A recent trend toward the miniaturization of electronic instruments requires a high energy density of cell. To satisfy such a requirement, a variety of secondary cells have been proposed. In such a cell, there is a non-aqueous electrolyte cell using lithium, which has been extensively studied for practical utility.
However, for the practical applications of the non-aqueous electrolyte cell, the following disadvantages are involved in the use of lithium metal as a negative electrode.
(1) Charging takes a time as long as 5-10 hours with poor quick charging properties.
(2) Short cycle life.
It is accepted that these are all ascribed to the lithium metal, resulting from the change in shape of the lithium negative electrode, the formation of dendrite and the inactivation of lithium accompanied by the repetition of charging and discharging.
One measure for solving the above problem, there has been proposed the use of a negative electrode wherein lithium is not used as a single material but is doped with a carbonaceous material. This makes use of easy electrochemical formation of a carbon layer compound of lithium. For instance, when a carbonaceous material used as a negative electrode is charged in non-aqueous electrolyte solution, lithium in the positive electrode is electrochemically doped in the interlayers of the negative carbon. The lithium-doped carbon acts as a lithium electrode and the lithium is de-doped from the carbon interlayers upon discharge, returning to the positive electrode.
The electric capacity per unit weight of the carbonaceous material is determined depending on the amount of doped lithium. In order to increase the charge and discharge capacity, it is desirable to increase the amount of doped lithium as large as possible. (The theoretical upper limit is a rate of one Li atom per 6 carbon atoms.)
Hitherto, the carbonaceous material used as the negative electrode of this type of cell includes carbonaceous materials obtained from organic materials as is known, for example, from Japanese Laid-open Patent Application No. 62-122066 or No. 62-90863.
However, the amount of doped lithium in the hitherto known carbonaceous materials is not so high that it is only approximately half the theoretical.